Coated paper having electroconductive properties and process of making the same



United States Patent 3,479,215 COATED PAPER HAVING ELECTROCONDUCTIVE SXIOIIIEERTIES AND PROCESS OF MAKING THE Giancarlo Cavagna, Adelphi, Md., and William C. Walker, New York, N.Y., assignors to Westvaco Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed Oct. 14, 1966, Ser. No. 586,615 Int. Cl. B32b 27/10; Htllb 1/06; C08g 41/00 US. Cl. 117-201 9 Claims ABSTRACT OF THE DISCLOSURE Polyethylenimine is quaternized to produce an electroconductive polymer. The quaternized polyethylenimine is applied to paper to produce a recording element having an electroconductive surface adapted for electrographic printing.

This invention relates to the manufacture of paper having electroconductive properties useful when the paper is employed as a recording element in an electrographic printing process. More particularly, this invention relates to paper having an electroconductive surface adapted for electrographic printing and containing a polymer comprised of quaternized polyethylenimine.

In the manufacture of paper having a printing surface adapted for electrographic printing, it previously has been found difiicult to provide the paper with a coating of an electroconductive substance that is dispersible in water and has the necessary electroconductive properties. One group of compounds which has been found to be useful comprises a polymer consisting of polymerized vinylbenzyl quaternary ammonium compound, disclosed in US. Patent 3,011,918 to Silvernail et a1. However, paper coated with these compounds suffers from an objectionably strong amine-like odor which can only be reduced by lowering the pH of the coating toa very acid range in which corrosion of the coating apparatus is promoted. Further, these compounds are expensive and a more economical, as well as satisfactory, electroconductive polymer has been sought.

It has now been discovered in accordance with the present invention that polymers of quaternized polyethylenimine can be prepared economically and can readily be an electroconductive surface and possessing electroconductive properties over a wide relative humidity range. Paper coated with quaternized polyethylenimine, in accordance with the present invention, has a surface resistivity ranging from about 1 10 to l 10 ohms per per square centimeter over a relative humidity range of about 8 to 75%. It is known in the industry that a product, like the ones described herein, should have a surface resistivity of no more than about l to 1-1-10 ohms square centimeter over a relatively humidity range of about 25 to 80% in order to be useful in electrographic printing processes.

Polyethylenimine, a polymer of ethylene imine, is quaternized with any alkylating agent, such as an alkyl halide like methyl chloride or an alkyl sulfate like dimethyl sulfate, or it may be quaternized with a substituted aryl halide, such as 1-chloro-2, 4-dinitrobenzene, or with an alkyl-aryl halide, such as benzyl chloride. The quaternization reaction can take place in many reaction solvents such as alcohols, ketones, and other hydrocarbons. but a reaction solvent is not required. The quaternization reaction goes well in water, in which polyethylenimine is soluble, if the alkylating agent is not extremely reactive with water, and water is the preferred reaction solvent for purposes of the present invention. Since alkyl chlorides are found to give better conductive properties than 3,479,215 Patented Nov. 18, 1969 the heavier iodides or bromides, the chlorides are the preferred halides for the quaternization reaction, but the other halides may be used.

From the above, it can be seen that the quaternized polyethylenimine of the present invention may be represented by the following general formula:

wherein R represents a radical selected from the group consisting of alkyl, substituted aryl, and alkyl-aryl hydrocarbon radicals, R represents a radical selected from the group consisting of quaternized polyethylenimine of the above general formula, and alkyl, substituted aryl, and alkyl-aryl hydrocarbon radicals, and X is an anion selected from the group consisting of chloride, bromide, iodide, and sulfate.

The molecular weight of the polyethylenimine before quaternization has little effect on the conductive properties of the polymer after quaternization, but molecular weight does affect the viscosity of the coating containing the quaternized polyethylenimine. Thus, the choice of the molecular weight of the material to be used is determined by the desired final viscosity. We have employed polyethylenimines having molecular weights ranging from 1200 to 100,000, and having applied continuous films of coatings containing these polymers to paper without any operational difficulties.

The polyethylenimine is preferably reacted with an alkylating agent such as methyl chloride by stirring together the polymer and alkylating agent at a temperature equal to or higher than room temperature. Temperatures between 40 to C. have been found preferable. The pH of the reaction medium, which tends to drop, is preferably kept on the alkaline side, between about 8 and 9, in order to achieve higher yields of the quaternized polymer. The reaction is complete after several hours, and the product can be used in solution, as it is, or it can be dried. If the quaternized polymer is used in solution, the pH can be adjusted towards neutral with an acid, such a hydrochloric acid. If the quaternized polymer is dried before use, it can be neutralized either before drying or before its use when it is put into solution. We have found that the alkaline product performs satisfactorily as an electroconductive coating, but the polymer, neutralized after quaternization, exhibits even higher conductive properties.

Instead of maintaining the reaction medium at a pH between about 8 and 9, as mentioned above, the reaction between polyethylenimine and the alkalating agent may be allowed to proceed without pH adjustment. Under these conditions, pH drops from the natural alkaline level of the polyethylenimine to about neutral, and quaternization of at least about 50% of the available nitrogen atoms in the polymer can be achieved. In any event, regardless of whether or not the pH is adjusted during the reaction, the number of quaternary ammonium groups present should be at least about for every 1,000 atoms forming the polymeric chain in order that the quaternized polyethylenimine possesses satisfactory conductive properties.

In preparing paper having an electroconductive surface containing the electroconductive polymers of this invention, the polymers can be applied as a continuous film to paper by coating, spraying, dipping, brushing or by any other suitable application. The quaternized polymer is preferably applied to paper, as a coating, in the form of an aqueous dispersion or solution. The coated paper is then dried to produce a product having an electroconductive surface comprised of a continuous film of the quaternized polyethylenimine. Standard coating devices, such as a blade coater, have been used, and the coated paper has been dried by the use of radiant energy from heat lamps. The electroconductive polymer has been applied at coat weights ranging from about 0.5 to 29.0 pounds of coating per ream (500 sheets, 25 x 38 inches), with a preferred coat weight being about 2 pounds per ream We have found that other polymers may be mixed with quaternized polyethylenimine before application of the coating to a paper substrate. A polymer, such as starch, may be used as a component of the coating formulation to prevent strike-through of the electroconductive polymer by keeping the latter on the surface of the paper. While the use of a polymer such as starch adds very little to the electroconductive properties of the coated paper, it may be used for the above indicated purpose. The starchquaternized polyethylenimine coating may contain any proportion of starch up to about a weight ratio of about 1 to 1 with the electroconductive polymer.

The coated paper is tested for surface resistivity by a procedure substantially like that described in Standard Methods of Test for Insulation Resistance of Electrical Insulating Materials, A.S.T.M. designation: D257-46.

In the actual practice of preparing a printing surface for an electrographic printing process, a coating of a photoconductive material, such as zinc oxide, is applied over the electroconductive surface containing the electroconductive polymers of this invention.

The manufacture and utilization of quaternized polyethylenimine, in accordance with the present invention, is illustrated in the following examples which are not to be construed as limiting the scope of the invention:

EXAMPLE 1 In a reaction vessel provided with a stirrer, thermometer, and condenser, 32.25 parts by weight of dry polyethylenimine, having a molecular weight of about 1200, were mixed with 600 parts by weight of water. The temperature of the solution was raised to 60 C., and then methyl chloride was bubbled through the solution. Throughout the quaternization reaction, the pH, which tends to drop, was maintained between about 8 and 9 by adding aqueous sodium hydroxide. After about 80 parts by weight of methyl chloride were added to the solution, the quaternization reaction was allowed to continue for about 10 hours. The reaction mixture was then freeze dried by using the Well known Virtis Unitrap apparatus, and about 90 parts by weight of dry quaternized polyethylenimine was obtained.

Part of the dried quaternized polyethylenimine was dissolved in water to form a 30% solids coating. A sheet of bleached sulfate paper, having a basis weight of about 50 pounds per ream (500 sheets, 25 x 38 inches), was coated on one side with a continuous film of the coating, using a No. 26 Meyer rod, to produce a coat weight of about 8.3 pounds per ream. The coated paper was dried by radiant energy, and then four test samples were cut from the polymer coated paper. These test samples were conditioned in air having a relative humidity of about 8%, at a temperature of about 25 C., for about 24 hours. The test samples were then tested for surface resistivity, and the samples exhibited an average surface resistivity of about 43x10 ohms per square centimeter. The polymer coated paper had an electroconductive surface adapted for electrographic printing.

EXAMPLE 2 The procedure of Example 1 was repeated, except that the 30% solids coating comprising quaternized polyethylenimine was neutralized to pH 6.8 with hydrochloric acid before the coating was applied to the paper. The paper was then coated, as in Example 1, with about 6.0 pounds per ream of the coating, dried, and four test samples were cut from the coated paper and conditioned similarly as in Example 1. When tested for resistivity, the polymer coated samples had an average surface resistivity of about 1.2 10 ohms per square centimeter, The polymer coated 4 paper had an electroconductive surface adapted for electrographic printing.

EXAMPLE 3 Example 1 was repeated to obtain the alkaline product. In accordance with our findings that another polymer can be added to the quaternized polyethylenimine for the purpose of keeping the conductive polymer on the surface of the coated substrate, and thereby prevent strike-through of the conductive polymer, a starch-quaternized polyethylenimine mixture was prepared. First, corn starch was gelled by cooking a 15% by weight starch-Water mixture for 15 minutes at 82 C. Then a coating was prepared comprising a mixture of 50% by weight of the gelled starch and 50% by weight of a 15% solution in water of the quaternized polyethylenimine.

The starch-quaternized polyethylenimine coating was applied to paper at a coat weight of about 5.2 pounds per ream (about 2.6 pounds per ream of the electroconductive polymer), and the coated paper was dried, conditioned at 8% relative humidity, and tested as in Example l. The average surface resistivity of the test samples was about 1.8)(10 ohms per square centimeter. The coated paper met the standards of the industry for use in the electrographic printing processes.

EXAMPLE 4 The procedure of Example 1 was repeated, and the test samples, which had a coat weight of about 22.5 pounds per ream, were tested as in Example 1 after being conditioned in air having a relative humidity of about 75 The average surface resistivity of the test samples was about 3.1 10 ohms per square centimeter.

EXAMPLE 5 The procedure of Example 2 was repeated, and the test samples, which had a coat weight of about 12.5 pounds per ream, were tested as in Example 2 after being conditioned in air having a relative humidity of about 75%. The average surface resistivity of the test samples was about 3..1 10 ohms per square centimeter.

EXAMPLE 6 The procedure of Example 3 was repeated, and the test samples, which were coated with about 7.4 pounds per ream of the starch-quaternized polyethylenimine coating (about 3.7 pounds per ream of the electroconductive polymer), were tested as in Example 3 after being conditioned in air having a relative humidity of about 75 The" average surface resistivity of the test samples was about 7.0 10 ohms per square centimeter.

EXAMPLE 7 The procedure of Example 4 was repeated, except that the polyethylenimine employed had a molecular weight of'about 40,000 to 60,000. The test samples were coated with about 4.5 pounds per ream of the coating, dried, conditioned in air having a relative humidity of about 75% and then tested as in Example 4. The average surface resistivity of the test samples was about 5.0 10 ohms per square centimeter.

EXAMPLE 8 The procedure of Example 5 was repeated, except that the polyethylenimine employed had a molecular weight of about 40,000 to 60,000. The test samples were coated withabout 2.6 pounds per ream of the coating, dried, conditioned in air having a relative humidity of about 75%, then tested as in Example 5. The average surface res st vity of the test sample was about 5.6 10 ohm per square centimeter.

EXAMPLE 9 In a reaction vessel provided with a stirrer, thermometer, and condenser, 4.5 parts by weight of methyl iodide were dissolved 1n 450 parts by weight of water. The solu.

tion was heated to about 40 C. and, with stirring, 1.3 parts by weight of polyethylenimine, having a molecular weight of about 1200, dissolved in about 65 parts by weight of water, were added dropwise to the methyl iodide-water solution. The pH was maintained between 8 and 9 throughout the quaternization reaction by the addition of aqueous sodium hydroxide. The reaction mixture was kept at about 40 C. for about 4 hours and then at about 25 C. for an additional 48 hours. The reaction mixture was then freeze dried to recover the quaternized polyethylenimine.

A coating of 15% solids concentration, comprising starch and quaternized polyethylenimine in the weight ratio of 1 to 1, as in Example 3, was prepared. Paper of about 50 pounds per ream basis weight was coated with about 7.4 pounds per ream of the coating (about 3.7 pounds per ream of the electroconductive polymer), then dried, and four test samples were cut from the paper, conditioned at about 15% relative humidity at 25 C., and then tested for resistivity. The average surface resistivity of the test samples was about 1.0 ohms per square centimeter.

EXAMPLE 10 The procedure of Example 9 Was repeated, except that the test samples were conditioned at about 75% relative humidity. The average surface resistivity of the samples was about 4.0x 10 ohms per square centimeter. By comparing Examples 8 and 9 with Examples 3 and 6 above, it can be seen that methyl iodide can be used as an alkylating agent for the polyethylenimine, but it is not quite as effective as methyl chloride in providing electroconductive properties at high or low relative humidities.

EXAMPLE 11 The procedure of Example 9 was repeated, except that the polyethylenimine used had a molecular weight of about 50,000 to 100,000. The test samples, which had a coat weight of about 5.0 pounds per ream of the starchquaternized polyethylenimine coating (about 2.5 pounds per ream of the electroconductive polymer) and were conditioned in air having a relative humidity of about exhibited an average surface resistivity of about 3.4)(10 ohms per square centimeter.

EXAMPLE 12 The procedure of Example 11 was repeated, except that the test samples were conditioned at about 75% relative humidity. The average surface resistivity of the samples was about 1.1x 10 ohms per square centimeter.

From Examples 11 and 12, it can be seen that the quaternized high molecular weight polyethylenimine provided good electroconductive properties at high humidity. Further tests have shown that if the high molecular weight polyethylenimine is quaternized with methyl chloride, rather than with methyl iodide as in Example 11, the quaternized polyethylenimine meets the industry requirements for electroconductivity at low relative humidity.

EXAMPLE 13 A reaction vessel provided with a stirrer, thermometer, and condenser was charged with 64.5 parts by weight of a 50% by weight water solution of polyethylenimine (32.25 parts polyethylenimine) having a molecular weight of about 1200. After the addition of another 500 parts by weight of water, and with stirring, the temperature was raised to about 70 C., and then methyl chloride was bubbled through the solution. After about 3 /2 hours, approximately 40 parts by weight of methyl chloride had been added, and the pH of the reaction medium had dropped from about 9.4 to 6.9 during the alkylation reaction. The reaction mixture was then freeze dried and about 58 parts by weight of dry quaternized polyethylenimine was obtained.

A coating of 50% solids concentration, comprising the quaternized polyethylenimine in W816i, was prepared.

Paper of about 50 pounds basis weight was coated with about 29.0 pounds per ream of the coating, then dried, and four test samples were cut from the paper, conditioned at about 11% relative humidity at about 25 C. When tested, the samples exhibited an average surface resistivity of about 3.5 l0 ohms per square centimeter. The polymer coated paper met the standards of the industry for use in electrographic printing processes.

From all of the above examples, it is apparent that polyethylenimines having molecular weights ranging from about 1200 to 100,000 can be quaternized to produce polyethylenimine quaternary ammonium compounds which can be employed as coatings to impart good electroconductivity properties to paper.

Various changes may be made in the illustrative examples specifically set forth above without departing from the spirit of our invention or the scope of the appended claims.

We claim:

1. Paper having an electroconductive surface adapted for electrographic printing and containing on said surface a continuous coating comprising an electroconductive water-dispersi-ble polymer, said polymer comprising quaternized polyethylenimine having at least about quaternary ammonium groups for each 1000 atoms forming the polymeric chain.

2. Paper according to claim 1 further characterized in that said polyethylenimine has a molecular weight between about 1200 and 100,000 before quaternization.

3. Paper according to claim 1 further characterized in that said polymer is present on said surface in an amount from about 0.5 to 29.0 pounds per ream of paper.

4. Paper according to claim 1 further characterized in that said coating further comprises starch in an amount by weight up to about 1 part starch per part of quaternized polyethylenimine.

5. The process of making paper having an electroconductive surface adapted for electrographic printing comprising the steps of applying to at least one surface of a paper substrate a coating of an aqueous dispersion containing quaternized polyethylenimine, said quaternized polyethylenimine having at least about 100 quaternary ammonium groups for each 1000 atoms forming the polymeric chain, and drying the coated paper, whereby said dried paper contains said quaternized polyethylenimine on said surface in the form of a continuous electroconductive coating in an amount from about 0.5 to 29.0 pounds per ream of paper.

6. The process of claim 5 which, before said step of applying the aqueous dispersion to said surface, comprises the step of adding starch to the aqueous dispersion in an amount by weight up to about 1 part starch per part of quaternized polyethylenimine in the aqueous dispersion.

7. The process of claim 5 which, prior to said step of applying the aqueous dispersion to said surface, comprises the steps of preparing an aqueous dispersion of quaternized polyethylenimine, and adjusting the pH of the aqueous dispersion to about neutral.

8. Paper having an electroconductive surface adapted for electrographic printing and containing on said surface a continuous coating comprising an electroconductive water-dispersible polymer, said polymer comprising quaternized polyethylenimine having the general formula:

L It. 1.

wherein R represents a radical selected from the group consisting of alkyl, substituted aryl, and alkyl-aryl hydrocarbon radicals, R represents a radical selected from the group consisting of quaternized polyethylenimine of the above general formula, and alkyl, substituted aryl, and alkyl-aryl hydrocarbon radicals, and X is an anion selected from the group consisting of chloride, bromide, iodide, and sulfate, said quaternized polyethylenimine having at least about 100 quaternary ammonium groups for each 1000 atoms forming the polymeric chain.

9. Paper having an electroconductive surface adapted for electrographic printing and containing on said surface a continuous coating of an electroconductive water-dispersible polymer comprising quaternized polyethylenimine having the general formula:

wherein R is a methyl group, R represents a member selected from the group consisting of quaternized polyethylenimine of the above general formula and a methyl group, and X is a chloride anion.

References Cited UNITED STATES PATENTS WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R.

" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 1 215 Dated 7 November 18, 1969 Inventor) Giancarlo A. Cavagna and William C, Walker It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 1, line 33, after "of' insert --a--; line 52, delete "per"; line 55,

for "l 10 read --1 x 10 line 56, before "square" insert -per--, and for "relatively" read --relative--. Column 4, line 25, delete "the"; line 69, for "ohm" read -ohms-.

SIGNED AND SEALED MAY 1 21970 E- Attest:

mm m. mmlr- WILLIAM E. mm, .m. A g Omar Oomissionar of Patcnta 

